Ethylene polymers may be formed in high-pressure polymerization processes. Individual steps in such processes are the compression of the reaction mixture to the reactor pressure, introduction of an initiator in at least one stage, polymerization while removing exothermic heat of reaction, product separation and further processing. For said processes, different types of reactor design may be used. One class of reactors to form low density ethylene-based polymers are tubular reactors.
A common set-up of a plant for preparing low density polyethylene comprises, besides the polymerization reactor, further equipment. The reaction components may be pressurized by a combination of two compressors, a primary compressor and a secondary compressor. At the end of the polymerization sequence, a high-pressure polymerization unit may further include apparatuses like extruders and granulators for pelletizing the obtained polymer. Furthermore, such a polymerization unit may also comprise means for feeding monomers and comonomers, free-radical initiators, chain transfer agents or other substances at one or more positions to the polymerization reaction. A process and an apparatus for the manufacture of ethylene polymers and copolymers under high pressures are for example disclosed in WO 2007/018871 A1.
Because tubular rectors for manufacturing LDPE are too long to be constructed as one long straight tube, the tubular reactor has to be spatially arranged. A common arrangement is that the tubular reactor is helically constructed; that means, the compressed reaction mixture advances in a kind of spiraling movement. One possibility of implementing such a design is that the tubular rector is located within a rectangular chamber and the helically arranged reactor tubes are attached to the walls of the chamber. Another possibility is to erect a scaffold and have the tubular rector spiraling along the scaffold.
The polymerization process in a LDPE reactor may be carried out at temperatures from 100° C. to 350° C. and high pressures which can reach 350 MPa. Such high temperatures and pressures require special technology for the process to be handled in a safe and reliable manner. Technical issues in handling ethylene at high pressures are, for example, described in Chem. Ing. Tech. 67 (1995), pages 862 to 864. It is stated that ethylene decomposes rapidly in an explosive manner under certain temperature and pressure conditions to give soot, methane and hydrogen. This undesired reaction occurs repeatedly in the high-pressure polymerization of ethylene. The drastic increase in pressure and temperature associated therewith represents a considerable potential risk for the operational safety of the production plants.
A possible solution for preventing a drastic increase in pressure and temperature of this type involves installing rupture discs or emergency pressure-relief valves. U.S. Pat. No. 4,255,387 discloses a high-pressure tubular reactor containing a plurality of tubular sections interconnected in series by means of connection devices, which reactor contains one or more reaction zones. Within each reaction zone, a single rupture disc device is positioned from about 24 to about 40 feet downstream from the reaction zone inlet. WO 02/01308 A2, for example, discloses a specific hydraulically controlled pressure relief valve which allows a particularly fast opening of the pressure relief valve in case of sudden changes in pressure or temperature. It is technically possible to handle such thermal runaways or explosive decompositions of ethylene within the polymerization reactor, although these situations should be avoided since thermal runaways or explosive decompositions of ethylene within the polymerization reactor lead to a shut-down of the polymerization plant with a possible emission of ethylene into the environment and loss of production.
Nonetheless, since it is not always possible to prevent thermal runaways or explosive decomposition, the equipment of the manufacturing plant has to withstand the mechanical forces which occur when an emergency shut-down is carried out. Furthermore, the equipment should allow for compensating the forces which might apply when the plant is brought into operation or shut down; i.e. when pressure and temperature are changed between operational state and ambient conditions.
There is accordingly a need to provide an improved design for arranging a tubular reactor of a manufacturing plant for high-pressure ethylene polymerization. The tubular reactor should allow efficient preparation of ethylene polymers and the costs of building, installing and operating the manufacturing plant should be reduced. It has hence to be ensured that the parts of the manufacturing plant can withstand the forces which might apply with respect to putting into operation or shutting down the manufacturing plant, even in emergency situations, and to compensate and endure, for example, pressure fluctuations and vibrations which might occur during regular operation of the manufacturing plant.